Process control systems, like those used in chemical, petroleum or other processes, typically include one or more centralized process controllers communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital and/or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches and/or transmitters (e.g., temperature sensors, pressure sensors and flow rate sensors), perform functions within the process such as opening and/or closing valves and/or measuring process parameters. The process controllers receive signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices, use this information to implement control routines and then generate control signals that are sent over the buses and/or other communication paths to the field devices to control the operation of the process. Information from the field devices and the controllers may be made available to one or more applications executed by the operator workstation to enable an operator to perform desired functions with respect to the process, such as viewing the current state of the process, modifying the operation of the process, etc.
Many process control systems also include one or more application stations. Typically, these application stations are implemented using a personal computer, workstation, or the like that is communicatively coupled to the controllers, operator workstations, and other systems within the process control system via a local area network (LAN). Each application station may execute one or more software applications that perform campaign management functions, maintenance management functions, virtual control functions, diagnostic functions, real-time monitoring functions, safety-related functions, configuration functions, etc. within the process control system.
Some process control systems or portions thereof may present significant safety risks. For example, chemical processing plants, power plants, etc. may implement critical processes that, if not properly controlled and/or shut down rapidly using a predetermined shut down sequence, could result in significant damage to people, the environment and/or equipment. To address the safety risks associated with process control systems having such critical processes, many process control system providers offer products compliant with safety-related standards such as, for example, the International Electrotechnical Commission (IEC) 61508 standard and the IEC 61511 standard.
Process control systems that are compliant with one or more known safety-related standards are typically implemented using an SIS architecture. In SIS architectures, the controllers and field devices associated with the basic process control system, which is responsible for the continuous control of the overall process, are physically and logically separate from special-purpose field devices and other special-purpose control elements associated with the SIS, which is responsible for the performance of safety instrumented functions to ensure the safe shutdown of the process in response to control conditions that present a significant safety risk. In particular, compliance with many known safety-related standards requires a basic process control system to be supplemented with special-purpose control elements such as logic solvers, SIS controllers, safety certified field devices (e.g., sensors, final control elements such as, for example, pneumatically actuated valves and/or solenoid valves), data redundancy devices and routines (e.g., redundancy links, cyclical redundancy checks, etc.), and safety certified software or code (e.g., certified applications, function modules, function blocks, etc.).
Some process plants, in addition to process control valves, include a safety shut-off valve (e.g., an emergency shut-off valve) for each of the process control valves to quickly stop the flow of fluid in response to, for example, a system failure. These safety shut-off valves are controlled by the SIS. An example safety shut-off valve is a solenoid valve where the SIS selects or controls a power supply of a solenoid to control the solenoid valve. When a solenoid is powered it and, thus, the solenoid valve will be in a first position or state (e.g., open). When power is removed from the solenoid, the solenoid and, thus, the solenoid valve will be in a second position or state (e.g., closed). Solenoid valves are typically of two types: fail-to-open, for which the valve is open when power is removed from the solenoid, and fail-to-close, for which the valve is closed when power is removed from the solenoid. Example solenoid valves include the EF8316 and EF8310 solenoid valves manufactured by Asco® Valve, Inc.